Recently, as existing energy resources like oil and coal and the like are expected to be exhausted, much attention is increasingly paid to alternative energy sources which can be used in place of the existing energy sources. As an alternative energy sources, sunlight energy is abundant and has no environmental pollution. Therefore, more and more attention is paid to the sunlight energy.
A photovoltaic module converting sunlight energy into electrical energy has a junction structure of a p-type semiconductor and an n-type semiconductor. When light is incident on the photovoltaic module, an electron with a negative electric charge and a -hole with a positive electric charge are generated by interaction between the light and a material constituting the semiconductor of the photovoltaic module. Then, electric current flows while the electron and the hole move.
Depending on the thickness of the semiconductor of the photovoltaic module, the photovoltaic module is classified into a bulk type photovoltaic module and a thin-film type photovoltaic module. The thin-film type photovoltaic module includes a photovoltaic material layer of which the thickness is equal to or less than from several tens of micrometers to several micrometers.
At present, a bulk type silicon photovoltaic module is mainly and widely used for ground power. However, the recent increase of the demand for the bulk type silicon photovoltaic module is now increasing its price due to the lack of its raw material.
Therefore, in recent times, providing an integrated thin-film photovoltaic module has become the most important issue, which has high energy conversion efficiency and can be mass produced at a low cost. However, a single-junction thin-film photovoltaic module is limited in its achievable performance. Accordingly, a double junction thin-film photovoltaic module or a triple junction thin-film photovoltaic module having a plurality of stacked unit cells has been developed, pursuing high stabilized efficiency. The double junction thin-film photovoltaic module and the triple junction thin-film photovoltaic module are called a tandem type photovoltaic module.
In addition to this, researches are now being devoted to an integration technology for the photovoltaic module in order to improve the efficiency of the thin-film photovoltaic module.
The integration technology for the photovoltaic module is to improve the photovoltaic conversion efficiency by reducing the interfacial resistance of a large area photovoltaic module. Up to the present day, the integration technology is performed by a conventional straight line type laser scribing. Since the conventional straight line type laser scribing is performed transversely to the movement direction of the electron, the moving distance of the electron becomes shorter, and then collection efficiency can be improved. However, a photovoltaic cell cannot generate photoelectron-motive force in a laser scribed portion. Therefore, an ineffective area of a solar cell is generated.
FIG. 1 is a perspective view of a thin-film photovoltaic module manufactured by a conventional straight line type laser scribing.
A lower electrode 200, a photoelectric conversion layer 300 and an upper electrode 400 are sequentially formed on a substrate 100. A lower electrode separation groove P1 penetrating through the lower electrode 200 is formed to prevent a short-circuit of the lower electrodes 200. A separation groove P2 is formed to penetrate through the photoelectric conversion layer 300. An upper separation groove P3 is formed to penetrate through the photoelectric conversion layer 300 and the upper electrode 400. Unit cells UC1 and UC2 are formed by the upper separation groove P3.
Here, the photoelectric conversion layer 300 may have a structure formed by stacking a plurality of unit cell layers. The unit cell layer is a basic unit layer capable of performing photoelectric conversion. For example, the photoelectric conversion layer 300 may include two stacked unit cell layers or three stacked unit cell layers. That is, each of the unit cells UC1 and UC2 may have a structure formed by stacking a plurality of the unit cell layers. The photoelectric conversion layer 300 may include a silicon alloy intermediate reflector such as silicon oxide, silicon nitride and silicon carbide in order to maximize light trapping effect by enhancing internal reflection.
Adjacent unit cells UC1 and UC2 are connected in series to each other by the separation groove P2 connecting the upper electrode 400 with the lower electrode 200 and functioning as a path of the electron. In other words, the lower electrode 200 of a first unit cell UC1 is connected to the upper electrode 400 of a second unit cell UC2 by the separation groove P2, so that the first unit cell UC1 is connected in series to the second unit cell UC2. All of the adjacent unit cells can be connected in series to each other.
As shown in FIG. 1, the lower electrode separation groove P1, the separation groove P2 and the upper separation groove P3 are formed respectively by a laser scribing which follows straight lines 210, 310 and 410.
Here, portions formed through the extension of the lower electrode separation groove P1, the separation groove P2 and the upper separation groove P3 correspond to ineffective areas incapable of performing energy conversion. In general, a gap between the lower electrode separation groove P1 and the upper separation groove P3 is from about 200 μm to about 300 μm.
The ineffective area is generated by the laser scribing in every gap between the unit photovoltaic cells connected in series to each other.
In the photovoltaic module, when a remaining area which is obtained by removing semiconductor and conductor for the purpose of edge isolation and performs the photoelectric conversion is referred to as an effective area, a ratio of the ineffective area by the laser scribing for series connection to the effective area is from about 2.5% to about 5.0%.
Accordingly, for the purpose of manufacturing the thin-film photovoltaic module having high efficiency, an integrated thin-film photovoltaic module and a manufacturing method thereof are required, which is able to improve photovoltaic conversion efficiency of the module by reducing the ineffective area through the minimization of a portion on which the laser scribing is performed. Further, in order to obtain high energy conversion efficiency, an integration technology is needed to be applied to the tandem type photovoltaic module.